This invention relates in general to electrostatography and, more specifically, to an electrostatographic imaging member having a charge transport layer comprising a mixture of a first hole transporting material and a different second hole transporting material containing only one long chain alkyl ester group.
In the art of xerography, a xerographic plate comprising a photoconductive insulating layer is imaged by first uniformly depositing a uniform electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electrostatically attractable marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography may comprise at least two electrically operative layers. This type of composite photoconductive layer is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent as having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer capable of photogenerating electrons and injecting the photogenerated electrons into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive materials exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous dispersions of conductive material in binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990 provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and therefore developed with finely developed electrostatically attractable marking particles.
If a thin, flat, biaxially oriented polyethylene terephthalate (e.g. 3 mil thick PET) web is solution coated with a thick imaging layer comprised of 50 percent by weight polycarbonate (e.g. Makrolon) and 50 percent by weight aromatic diamine, the web tends to curl. The curling begins when the coating solvent evaporates, due to the dimensional contraction of the applied coating from the point in time when the applied charge transport coating solidifies and adheres to the underlying surface. Once this solidification and adhesion point is reached, further evaporation of the coating solvent causes continued shrinking of the applied coating layer due to the volume contraction. Since the polyethylene terephthalate substrate does not undergo any dimensional changes, the continued shrinkage of the charge transport layer coating causes the edges of the coated web to curl toward the coated side of the substrate. This shrinking occurs isotropically, i.e., three-dimensionally. In other words, from the point in time when the applied coating has reached a solid state and is anchored at the interface with the underlying support layer, continued shrinking of the applied coating causes dimensional decreases in the applied coating which in turn builds up internal tension stress and, therefore, forces the entire coated structure to curl toward the dry applied charge transport layer coating. Internal tension is undesirable because it causes distortion of the imaging surface of the photoconductive member. This can cause different segments of the photoreceptor surface to be located at different distances from charging devices, developer applicators, toner image receiving members and the like during the electrophotographic imaging process, thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. A free standing sample of such an imaging member can spontaneously form a roll as small as 3.8 cm in diameter and requires considerable tension to flatten the imaging member against the surface of a separate supporting device. Where the supporting device comprises a large flat area for full frame flash exposure, the imaging member may tear before sufficient flatness can be achieved. Moreover, constant flexing of multilayered photoreceptor belts during cycling can cause stress cracks to form due to fatigue. These cracks print out on the final electrophotographic copy. Premature failure due to fatigue prohibits use of these belts in designs utilizing small support roller sizes (e.g. 19 mm or smaller) which are desirable for effective auto paper stripping. Coatings may be applied to the side of the supporting substrate opposite the electrically active layer or layers to counteract the tendency to curl. However, such an anticurl coating requires an additional coating step on the side of the substrate opposite from the side where all the other coatings are applied. This additional coating operation normally requires that a substrate web be unrolled an additional time merely to apply the anticurl layer. Also, many of the solvents utilized to apply the anticurl layer require additional steps and solvent recovery equipment to minimize solvent pollution of the atmosphere. Further, equipment required to apply the anticurl coating must be cleaned with solvent and refurbished from time to time. The additional coating operations raise the cost of the photoreceptor, increase manufacturing time, decrease production throughput, and increase the likelihood that the photoreceptor will be damaged by the additional handling. In addition, the anticurl backing layer can form bubbles during application which requires scrapping of that portion of the photoreceptor containing the bubbles. This in turn reduces total manufacturing yield. Also, difficulties have been encountered with these anticurl coatings. For example, photoreceptor curl can sometimes still be encountered due to a decrease in anticurl layer thickness resulting from wear in as few as 1,500 imaging cycles when the photoreceptor belt is exposed to stressful operating conditions of high temperature and high humidity. The curling of the photoreceptor is inherently caused by an imbalance of internal stresses between the electrically active layer and the anticurl coating. This can promote dynamic fatigue cracking, thereby shortening the mechanical life of the photoreceptor. Wear of the anticurl coating can also result in distortions which resemble ripples. These ripples are the most serious photoreceptor related problem in advanced precision imaging machines that demand precise tolerances. When ripples are present, it again results in different segments of the imaging surface of the photoconductive member being located at different distances from charging devices, developer applicators, toner image receiving members and the like during the electrophotographic imaging process thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. It is theorized that the anticurl backing layer is subjected to a highly abrasive environment including drive rollers, guiding rollers and especially stationary skid plates. In this environment, it wears rapidly during extended image cycling. This wear is non-uniform and leads to the distortions which resemble ripples. The wear process also produces debris which can form undesirable deposits on sensitive optics, corotron wires and the like. The anticurl backing layer is usually composed of material that is less wear resistant than the adjacent substrate layer, hence less debris would be generated in a photoreceptor device not needing an anticurl layer. Further, the anticurl coatings occasionally separate from the substrate during extended machine cycling and render the photoconductive imaging member unacceptable for forming quality images. Anticurl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Moreover, in electrostatographic imaging systems where transparency of the substrate and anticurl layer are necessary for rear exposure erase to activating electromagnetic radiation, any reduction of transparency due to the presence of an anticurl layer will cause a reduction in performance of the photoconductive imaging member. Although the reduction in transparency may in some cases be compensated for by increasing the intensity of the electromagnetic radiation, such an increase is generally undesirable due to the amount of heat generated as well as the greater costs necessary to achieve higher intensity.
Another property of significance in multilayer devices is the charge carrier mobility in the transport layer. Charge carrier mobilities determine the velocities at which the photoinjected carriers transit the transport layer. To achieve maximum discharge or sensitivity for a fixed exposure, the photoinjected carriers must transit the transport layer before the imagewise exposed region of the photoreceptor arrives at the development station. To the extent the carriers are still in transit when the exposed segment of the photoreceptor arrives at the development station, the discharge is reduced and hence the contrast potentials available for development are also reduced. For greater charge carrier mobility capabilities, it is normally necessary to increase the concentration of the active molecule transport compound dissolved or molecularly dispersed in the binder. Phase separation or crystallization sets an upper limit to the concentration of the transport molecules that can be dispersed in a binder. One way of increasing the solubility limit of the transport molecule is to attach long alkyl groups on to the transport molecules. However, these alkyl groups are "inactive" and do not transport charge. For a given concentration of the transport molecules, these side chains actually reduce the charge carrier mobilities. A second factor that reduces the charge carrier mobilities is the dipole content of the charge transport molecules, the side groups of the charge transport molecules, and the binder in which the molecules are dispersed. One prior art approach for reducing the curl (see U.S. Pat. No. 5,728,498 referenced below) involves an imaging member comprising hole transporting material containing at least two long chain alkyl carboxylate groups dissolved or molecularly dispersed in a film forming binder. The prior art suggests the use of these molecules containing long chain alkyl carboxylate groups dispersed in a binder or in combination with a conventional hole transport molecule. However, when in combination with the conventional transport molecule, the concentration of the molecule with the long chain alkyl carboxylate groups has to be considerably larger than 15 percent by weight based on the total weight of the layer in order to eliminate curl. Although curl is eliminated and these devices can be used in electrophotography, high speed electrophotography requires high charge carrier mobilities. Use of a large concentration of transporting material containing at least two long chain alkyl carboxylate groups results in a drop in mobilities because of the "inactive" long chains required to reduce curl as well as the dipole content of these long alkyl carboxylate groups.
Another shortcoming of the prior art is the propensity for deletion. Deletion requires special engineering solutions such as optimized airflows in and around corotrons. Reprographic machines containing multilayered organic photoconductors often employ corotrons or scorotrons to charge the photoconductor prior to imagewise exposure. During the operating lifetime of these photoconductors, they are subjected to corona effluents which include ozone, various oxides of nitrogen etc. It is believed that some of these oxides of nitrogen are converted to nitric acid in the presence of water molecules present in the ambient operating atmosphere. The top surface of the photoconductor is exposed to the nitric acid during operation of the machine and the photoconductor molecules at the very top surface of the transport layer are converted to what is believed to be the nitrated species of the molecules and these could form electrically conductive film. However, during operation of the machine, the cleaning subsystem continuously removes (by wear) a region of the top surface thereby preventing accumulation of the conductive species. Unfortunately, such is not the case when the machine is not operating (i.e., in the idle mode) between two large copy runs. During the idle mode between long copy runs, for example, runs for a 1000 copies, a specific segment of the photoreceptor comes to rest (parked) beneath the corotron that had been in operation during the long copy run. Although the high voltage to the corotron is turned off during the time period when the photoreceptor is parked, some effluents (i.e., nitric acid etc.) continue to be emitted from the corotron shield, corotron housing, etc. This effluent emission is concentrated in the region of the stationary photoreceptor parked directly underneath the corotron. The effluents render the surface region of the photoreceptor electrically conductive. When machine operation is resumed for the next copy run, a loss of resolution, and even deletion, is observed in the affected region of the photoreceptor. Thus, the corona induced changes primarily occur in the surface region of the charge transport layer. The problem of deletion may also occur as a loss of resolution during an extended copying run. The onset of loss of resolution depends on the type and number of corotrons employed and the airflow configuration within the machines. Although one of the prior art (see U.S. Pat. No. 6,025,102 referenced below) dealing with curl elimination accomplished curl elimination without sacrificing charge carrier mobility, it did not improve resistance to corona induced deletion.